src/librustc_trait_selection/traits/project.rs

   1 //! Code for projecting associated types out of trait references.
   2
   3 use super::elaborate_predicates;
   4 use super::specialization_graph;
   5 use super::translate_substs;
   6 use super::util;
   7 use super::MismatchedProjectionTypes;
   8 use super::Obligation;
   9 use super::ObligationCause;
  10 use super::PredicateObligation;
  11 use super::Selection;
  12 use super::SelectionContext;
  13 use super::SelectionError;
  14 use super::{
  15     ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  16     ImplSourceGeneratorData, ImplSourceUserDefinedData,
  17 };
  18 use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
  19
  20 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  21 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
  22 use crate::traits::error_reporting::InferCtxtExt;
  23 use rustc_data_structures::stack::ensure_sufficient_stack;
  24 use rustc_errors::ErrorReported;
  25 use rustc_hir::def_id::DefId;
  26 use rustc_hir::lang_items::{FnOnceOutputLangItem, FnOnceTraitLangItem, GeneratorTraitLangItem};
  27 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
  28 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
  29 use rustc_middle::ty::subst::Subst;
  30 use rustc_middle::ty::util::IntTypeExt;
  31 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
  32 use rustc_span::symbol::sym;
  33 use rustc_span::DUMMY_SP;
  34
  35 pub use rustc_middle::traits::Reveal;
  36
  37 pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
  38
  39 pub type ProjectionObligation<'tcx> = Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
  40
  41 pub type ProjectionTyObligation<'tcx> = Obligation<'tcx, ty::ProjectionTy<'tcx>>;
  42
  43 /// When attempting to resolve `<T as TraitRef>::Name` ...
  44 #[derive(Debug)]
  45 pub enum ProjectionTyError<'tcx> {
  46     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  47     TooManyCandidates,
  48
  49     /// ...an error occurred matching `T : TraitRef`
  50     TraitSelectionError(SelectionError<'tcx>),
  51 }
  52
  53 #[derive(PartialEq, Eq, Debug)]
  54 enum ProjectionTyCandidate<'tcx> {
  55     // from a where-clause in the env or object type
  56     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
  57
  58     // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
  59     TraitDef(ty::PolyProjectionPredicate<'tcx>),
  60
  61     // from a "impl" (or a "pseudo-impl" returned by select)
  62     Select(Selection<'tcx>),
  63 }
  64
  65 enum ProjectionTyCandidateSet<'tcx> {
  66     None,
  67     Single(ProjectionTyCandidate<'tcx>),
  68     Ambiguous,
  69     Error(SelectionError<'tcx>),
  70 }
  71
  72 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
  73     fn mark_ambiguous(&mut self) {
  74         *self = ProjectionTyCandidateSet::Ambiguous;
  75     }
  76
  77     fn mark_error(&mut self, err: SelectionError<'tcx>) {
  78         *self = ProjectionTyCandidateSet::Error(err);
  79     }
  80
  81     // Returns true if the push was successful, or false if the candidate
  82     // was discarded -- this could be because of ambiguity, or because
  83     // a higher-priority candidate is already there.
  84     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
  85         use self::ProjectionTyCandidate::*;
  86         use self::ProjectionTyCandidateSet::*;
  87
  88         // This wacky variable is just used to try and
  89         // make code readable and avoid confusing paths.
  90         // It is assigned a "value" of `()` only on those
  91         // paths in which we wish to convert `*self` to
  92         // ambiguous (and return false, because the candidate
  93         // was not used). On other paths, it is not assigned,
  94         // and hence if those paths *could* reach the code that
  95         // comes after the match, this fn would not compile.
  96         let convert_to_ambiguous;
  97
  98         match self {
  99             None => {
 100                 *self = Single(candidate);
 101                 return true;
 102             }
 103
 104             Single(current) => {
 105                 // Duplicates can happen inside ParamEnv. In the case, we
 106                 // perform a lazy deduplication.
 107                 if current == &candidate {
 108                     return false;
 109                 }
 110
 111                 // Prefer where-clauses. As in select, if there are multiple
 112                 // candidates, we prefer where-clause candidates over impls.  This
 113                 // may seem a bit surprising, since impls are the source of
 114                 // "truth" in some sense, but in fact some of the impls that SEEM
 115                 // applicable are not, because of nested obligations. Where
 116                 // clauses are the safer choice. See the comment on
 117                 // `select::SelectionCandidate` and #21974 for more details.
 118                 match (current, candidate) {
 119                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 120                     (ParamEnv(..), _) => return false,
 121                     (_, ParamEnv(..)) => unreachable!(),
 122                     (_, _) => convert_to_ambiguous = (),
 123                 }
 124             }
 125
 126             Ambiguous | Error(..) => {
 127                 return false;
 128             }
 129         }
 130
 131         // We only ever get here when we moved from a single candidate
 132         // to ambiguous.
 133         let () = convert_to_ambiguous;
 134         *self = Ambiguous;
 135         false
 136     }
 137 }
 138
 139 /// Evaluates constraints of the form:
 140 ///
 141 ///     for<...> <T as Trait>::U == V
 142 ///
 143 /// If successful, this may result in additional obligations. Also returns
 144 /// the projection cache key used to track these additional obligations.
 145 pub fn poly_project_and_unify_type<'cx, 'tcx>(
 146     selcx: &mut SelectionContext<'cx, 'tcx>,
 147     obligation: &PolyProjectionObligation<'tcx>,
 148 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 149     debug!("poly_project_and_unify_type(obligation={:?})", obligation);
 150
 151     let infcx = selcx.infcx();
 152     infcx.commit_if_ok(|_snapshot| {
 153         let (placeholder_predicate, _) =
 154             infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
 155
 156         let placeholder_obligation = obligation.with(placeholder_predicate);
 157         let result = project_and_unify_type(selcx, &placeholder_obligation)?;
 158         Ok(result)
 159     })
 160 }
 161
 162 /// Evaluates constraints of the form:
 163 ///
 164 ///     <T as Trait>::U == V
 165 ///
 166 /// If successful, this may result in additional obligations.
 167 fn project_and_unify_type<'cx, 'tcx>(
 168     selcx: &mut SelectionContext<'cx, 'tcx>,
 169     obligation: &ProjectionObligation<'tcx>,
 170 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 171     debug!("project_and_unify_type(obligation={:?})", obligation);
 172
 173     let mut obligations = vec![];
 174     let normalized_ty = match opt_normalize_projection_type(
 175         selcx,
 176         obligation.param_env,
 177         obligation.predicate.projection_ty,
 178         obligation.cause.clone(),
 179         obligation.recursion_depth,
 180         &mut obligations,
 181     ) {
 182         Some(n) => n,
 183         None => return Ok(None),
 184     };
 185
 186     debug!(
 187         "project_and_unify_type: normalized_ty={:?} obligations={:?}",
 188         normalized_ty, obligations
 189     );
 190
 191     let infcx = selcx.infcx();
 192     match infcx
 193         .at(&obligation.cause, obligation.param_env)
 194         .eq(normalized_ty, obligation.predicate.ty)
 195     {
 196         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 197             obligations.extend(inferred_obligations);
 198             Ok(Some(obligations))
 199         }
 200         Err(err) => {
 201             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 202             Err(MismatchedProjectionTypes { err })
 203         }
 204     }
 205 }
 206
 207 /// Normalizes any associated type projections in `value`, replacing
 208 /// them with a fully resolved type where possible. The return value
 209 /// combines the normalized result and any additional obligations that
 210 /// were incurred as result.
 211 pub fn normalize<'a, 'b, 'tcx, T>(
 212     selcx: &'a mut SelectionContext<'b, 'tcx>,
 213     param_env: ty::ParamEnv<'tcx>,
 214     cause: ObligationCause<'tcx>,
 215     value: &T,
 216 ) -> Normalized<'tcx, T>
 217 where
 218     T: TypeFoldable<'tcx>,
 219 {
 220     let mut obligations = Vec::new();
 221     let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
 222     Normalized { value, obligations }
 223 }
 224
 225 pub fn normalize_to<'a, 'b, 'tcx, T>(
 226     selcx: &'a mut SelectionContext<'b, 'tcx>,
 227     param_env: ty::ParamEnv<'tcx>,
 228     cause: ObligationCause<'tcx>,
 229     value: &T,
 230     obligations: &mut Vec<PredicateObligation<'tcx>>,
 231 ) -> T
 232 where
 233     T: TypeFoldable<'tcx>,
 234 {
 235     normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
 236 }
 237
 238 /// As `normalize`, but with a custom depth.
 239 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
 240     selcx: &'a mut SelectionContext<'b, 'tcx>,
 241     param_env: ty::ParamEnv<'tcx>,
 242     cause: ObligationCause<'tcx>,
 243     depth: usize,
 244     value: &T,
 245 ) -> Normalized<'tcx, T>
 246 where
 247     T: TypeFoldable<'tcx>,
 248 {
 249     let mut obligations = Vec::new();
 250     let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
 251     Normalized { value, obligations }
 252 }
 253
 254 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
 255     selcx: &'a mut SelectionContext<'b, 'tcx>,
 256     param_env: ty::ParamEnv<'tcx>,
 257     cause: ObligationCause<'tcx>,
 258     depth: usize,
 259     value: &T,
 260     obligations: &mut Vec<PredicateObligation<'tcx>>,
 261 ) -> T
 262 where
 263     T: TypeFoldable<'tcx>,
 264 {
 265     debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
 266     let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
 267     let result = ensure_sufficient_stack(|| normalizer.fold(value));
 268     debug!(
 269         "normalize_with_depth: depth={} result={:?} with {} obligations",
 270         depth,
 271         result,
 272         normalizer.obligations.len()
 273     );
 274     debug!("normalize_with_depth: depth={} obligations={:?}", depth, normalizer.obligations);
 275     result
 276 }
 277
 278 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
 279     selcx: &'a mut SelectionContext<'b, 'tcx>,
 280     param_env: ty::ParamEnv<'tcx>,
 281     cause: ObligationCause<'tcx>,
 282     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 283     depth: usize,
 284 }
 285
 286 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
 287     fn new(
 288         selcx: &'a mut SelectionContext<'b, 'tcx>,
 289         param_env: ty::ParamEnv<'tcx>,
 290         cause: ObligationCause<'tcx>,
 291         depth: usize,
 292         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 293     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
 294         AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
 295     }
 296
 297     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
 298         let value = self.selcx.infcx().resolve_vars_if_possible(value);
 299
 300         if !value.has_projections() { value } else { value.fold_with(self) }
 301     }
 302 }
 303
 304 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 305     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
 306         self.selcx.tcx()
 307     }
 308
 309     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 310         if !ty.has_projections() {
 311             return ty;
 312         }
 313         // We don't want to normalize associated types that occur inside of region
 314         // binders, because they may contain bound regions, and we can't cope with that.
 315         //
 316         // Example:
 317         //
 318         //     for<'a> fn(<T as Foo<&'a>>::A)
 319         //
 320         // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
 321         // normalize it when we instantiate those bound regions (which
 322         // should occur eventually).
 323
 324         let ty = ty.super_fold_with(self);
 325         match ty.kind {
 326             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
 327                 // (*)
 328                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 329                 match self.param_env.reveal() {
 330                     Reveal::UserFacing => ty,
 331
 332                     Reveal::All => {
 333                         let recursion_limit = self.tcx().sess.recursion_limit();
 334                         if !recursion_limit.value_within_limit(self.depth) {
 335                             let obligation = Obligation::with_depth(
 336                                 self.cause.clone(),
 337                                 recursion_limit.0,
 338                                 self.param_env,
 339                                 ty,
 340                             );
 341                             self.selcx.infcx().report_overflow_error(&obligation, true);
 342                         }
 343
 344                         let generic_ty = self.tcx().type_of(def_id);
 345                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 346                         self.depth += 1;
 347                         let folded_ty = self.fold_ty(concrete_ty);
 348                         self.depth -= 1;
 349                         folded_ty
 350                     }
 351                 }
 352             }
 353
 354             ty::Projection(ref data) if !data.has_escaping_bound_vars() => {
 355                 // (*)
 356
 357                 // (*) This is kind of hacky -- we need to be able to
 358                 // handle normalization within binders because
 359                 // otherwise we wind up a need to normalize when doing
 360                 // trait matching (since you can have a trait
 361                 // obligation like `for<'a> T::B: Fn(&'a i32)`), but
 362                 // we can't normalize with bound regions in scope. So
 363                 // far now we just ignore binders but only normalize
 364                 // if all bound regions are gone (and then we still
 365                 // have to renormalize whenever we instantiate a
 366                 // binder). It would be better to normalize in a
 367                 // binding-aware fashion.
 368
 369                 let normalized_ty = normalize_projection_type(
 370                     self.selcx,
 371                     self.param_env,
 372                     *data,
 373                     self.cause.clone(),
 374                     self.depth,
 375                     &mut self.obligations,
 376                 );
 377                 debug!(
 378                     "AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
 379                      now with {} obligations",
 380                     self.depth,
 381                     ty,
 382                     normalized_ty,
 383                     self.obligations.len()
 384                 );
 385                 normalized_ty
 386             }
 387
 388             _ => ty,
 389         }
 390     }
 391
 392     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 393         if self.selcx.tcx().lazy_normalization() {
 394             constant
 395         } else {
 396             let constant = constant.super_fold_with(self);
 397             constant.eval(self.selcx.tcx(), self.param_env)
 398         }
 399     }
 400 }
 401
 402 /// The guts of `normalize`: normalize a specific projection like `<T
 403 /// as Trait>::Item`. The result is always a type (and possibly
 404 /// additional obligations). If ambiguity arises, which implies that
 405 /// there are unresolved type variables in the projection, we will
 406 /// substitute a fresh type variable `$X` and generate a new
 407 /// obligation `<T as Trait>::Item == $X` for later.
 408 pub fn normalize_projection_type<'a, 'b, 'tcx>(
 409     selcx: &'a mut SelectionContext<'b, 'tcx>,
 410     param_env: ty::ParamEnv<'tcx>,
 411     projection_ty: ty::ProjectionTy<'tcx>,
 412     cause: ObligationCause<'tcx>,
 413     depth: usize,
 414     obligations: &mut Vec<PredicateObligation<'tcx>>,
 415 ) -> Ty<'tcx> {
 416     opt_normalize_projection_type(
 417         selcx,
 418         param_env,
 419         projection_ty,
 420         cause.clone(),
 421         depth,
 422         obligations,
 423     )
 424     .unwrap_or_else(move || {
 425         // if we bottom out in ambiguity, create a type variable
 426         // and a deferred predicate to resolve this when more type
 427         // information is available.
 428
 429         let tcx = selcx.infcx().tcx;
 430         let def_id = projection_ty.item_def_id;
 431         let ty_var = selcx.infcx().next_ty_var(TypeVariableOrigin {
 432             kind: TypeVariableOriginKind::NormalizeProjectionType,
 433             span: tcx.def_span(def_id),
 434         });
 435         let projection = ty::Binder::dummy(ty::ProjectionPredicate { projection_ty, ty: ty_var });
 436         let obligation =
 437             Obligation::with_depth(cause, depth + 1, param_env, projection.to_predicate(tcx));
 438         obligations.push(obligation);
 439         ty_var
 440     })
 441 }
 442
 443 /// The guts of `normalize`: normalize a specific projection like `<T
 444 /// as Trait>::Item`. The result is always a type (and possibly
 445 /// additional obligations). Returns `None` in the case of ambiguity,
 446 /// which indicates that there are unbound type variables.
 447 ///
 448 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 449 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 450 /// often immediately appended to another obligations vector. So now this
 451 /// function takes an obligations vector and appends to it directly, which is
 452 /// slightly uglier but avoids the need for an extra short-lived allocation.
 453 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 454     selcx: &'a mut SelectionContext<'b, 'tcx>,
 455     param_env: ty::ParamEnv<'tcx>,
 456     projection_ty: ty::ProjectionTy<'tcx>,
 457     cause: ObligationCause<'tcx>,
 458     depth: usize,
 459     obligations: &mut Vec<PredicateObligation<'tcx>>,
 460 ) -> Option<Ty<'tcx>> {
 461     let infcx = selcx.infcx();
 462
 463     let projection_ty = infcx.resolve_vars_if_possible(&projection_ty);
 464     let cache_key = ProjectionCacheKey::new(projection_ty);
 465
 466     debug!(
 467         "opt_normalize_projection_type(\
 468          projection_ty={:?}, \
 469          depth={})",
 470         projection_ty, depth
 471     );
 472
 473     // FIXME(#20304) For now, I am caching here, which is good, but it
 474     // means we don't capture the type variables that are created in
 475     // the case of ambiguity. Which means we may create a large stream
 476     // of such variables. OTOH, if we move the caching up a level, we
 477     // would not benefit from caching when proving `T: Trait<U=Foo>`
 478     // bounds. It might be the case that we want two distinct caches,
 479     // or else another kind of cache entry.
 480
 481     let cache_result = infcx.inner.borrow_mut().projection_cache().try_start(cache_key);
 482     match cache_result {
 483         Ok(()) => {}
 484         Err(ProjectionCacheEntry::Ambiguous) => {
 485             // If we found ambiguity the last time, that means we will continue
 486             // to do so until some type in the key changes (and we know it
 487             // hasn't, because we just fully resolved it).
 488             debug!(
 489                 "opt_normalize_projection_type: \
 490                  found cache entry: ambiguous"
 491             );
 492             return None;
 493         }
 494         Err(ProjectionCacheEntry::InProgress) => {
 495             // If while normalized A::B, we are asked to normalize
 496             // A::B, just return A::B itself. This is a conservative
 497             // answer, in the sense that A::B *is* clearly equivalent
 498             // to A::B, though there may be a better value we can
 499             // find.
 500
 501             // Under lazy normalization, this can arise when
 502             // bootstrapping.  That is, imagine an environment with a
 503             // where-clause like `A::B == u32`. Now, if we are asked
 504             // to normalize `A::B`, we will want to check the
 505             // where-clauses in scope. So we will try to unify `A::B`
 506             // with `A::B`, which can trigger a recursive
 507             // normalization. In that case, I think we will want this code:
 508             //
 509             // ```
 510             // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
 511             //                                    projection_ty.substs;
 512             // return Some(NormalizedTy { value: v, obligations: vec![] });
 513             // ```
 514
 515             debug!(
 516                 "opt_normalize_projection_type: \
 517                  found cache entry: in-progress"
 518             );
 519
 520             // But for now, let's classify this as an overflow:
 521             let recursion_limit = selcx.tcx().sess.recursion_limit();
 522             let obligation =
 523                 Obligation::with_depth(cause, recursion_limit.0, param_env, projection_ty);
 524             selcx.infcx().report_overflow_error(&obligation, false);
 525         }
 526         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 527             // This is the hottest path in this function.
 528             //
 529             // If we find the value in the cache, then return it along
 530             // with the obligations that went along with it. Note
 531             // that, when using a fulfillment context, these
 532             // obligations could in principle be ignored: they have
 533             // already been registered when the cache entry was
 534             // created (and hence the new ones will quickly be
 535             // discarded as duplicated). But when doing trait
 536             // evaluation this is not the case, and dropping the trait
 537             // evaluations can causes ICEs (e.g., #43132).
 538             debug!(
 539                 "opt_normalize_projection_type: \
 540                  found normalized ty `{:?}`",
 541                 ty
 542             );
 543
 544             // Once we have inferred everything we need to know, we
 545             // can ignore the `obligations` from that point on.
 546             if infcx.unresolved_type_vars(&ty.value).is_none() {
 547                 infcx.inner.borrow_mut().projection_cache().complete_normalized(cache_key, &ty);
 548             // No need to extend `obligations`.
 549             } else {
 550                 obligations.extend(ty.obligations);
 551             }
 552
 553             obligations.push(get_paranoid_cache_value_obligation(
 554                 infcx,
 555                 param_env,
 556                 projection_ty,
 557                 cause,
 558                 depth,
 559             ));
 560             return Some(ty.value);
 561         }
 562         Err(ProjectionCacheEntry::Error) => {
 563             debug!(
 564                 "opt_normalize_projection_type: \
 565                  found error"
 566             );
 567             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 568             obligations.extend(result.obligations);
 569             return Some(result.value);
 570         }
 571     }
 572
 573     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 574     match project_type(selcx, &obligation) {
 575         Ok(ProjectedTy::Progress(Progress {
 576             ty: projected_ty,
 577             obligations: mut projected_obligations,
 578         })) => {
 579             // if projection succeeded, then what we get out of this
 580             // is also non-normalized (consider: it was derived from
 581             // an impl, where-clause etc) and hence we must
 582             // re-normalize it
 583
 584             debug!(
 585                 "opt_normalize_projection_type: \
 586                  projected_ty={:?} \
 587                  depth={} \
 588                  projected_obligations={:?}",
 589                 projected_ty, depth, projected_obligations
 590             );
 591
 592             let result = if projected_ty.has_projections() {
 593                 let mut normalizer = AssocTypeNormalizer::new(
 594                     selcx,
 595                     param_env,
 596                     cause,
 597                     depth + 1,
 598                     &mut projected_obligations,
 599                 );
 600                 let normalized_ty = normalizer.fold(&projected_ty);
 601
 602                 debug!(
 603                     "opt_normalize_projection_type: \
 604                      normalized_ty={:?} depth={}",
 605                     normalized_ty, depth
 606                 );
 607
 608                 Normalized { value: normalized_ty, obligations: projected_obligations }
 609             } else {
 610                 Normalized { value: projected_ty, obligations: projected_obligations }
 611             };
 612
 613             let cache_value = prune_cache_value_obligations(infcx, &result);
 614             infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
 615             obligations.extend(result.obligations);
 616             Some(result.value)
 617         }
 618         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 619             debug!(
 620                 "opt_normalize_projection_type: \
 621                  projected_ty={:?} no progress",
 622                 projected_ty
 623             );
 624             let result = Normalized { value: projected_ty, obligations: vec![] };
 625             infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
 626             // No need to extend `obligations`.
 627             Some(result.value)
 628         }
 629         Err(ProjectionTyError::TooManyCandidates) => {
 630             debug!(
 631                 "opt_normalize_projection_type: \
 632                  too many candidates"
 633             );
 634             infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
 635             None
 636         }
 637         Err(ProjectionTyError::TraitSelectionError(_)) => {
 638             debug!("opt_normalize_projection_type: ERROR");
 639             // if we got an error processing the `T as Trait` part,
 640             // just return `ty::err` but add the obligation `T :
 641             // Trait`, which when processed will cause the error to be
 642             // reported later
 643
 644             infcx.inner.borrow_mut().projection_cache().error(cache_key);
 645             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 646             obligations.extend(result.obligations);
 647             Some(result.value)
 648         }
 649     }
 650 }
 651
 652 /// If there are unresolved type variables, then we need to include
 653 /// any subobligations that bind them, at least until those type
 654 /// variables are fully resolved.
 655 fn prune_cache_value_obligations<'a, 'tcx>(
 656     infcx: &'a InferCtxt<'a, 'tcx>,
 657     result: &NormalizedTy<'tcx>,
 658 ) -> NormalizedTy<'tcx> {
 659     if infcx.unresolved_type_vars(&result.value).is_none() {
 660         return NormalizedTy { value: result.value, obligations: vec![] };
 661     }
 662
 663     let mut obligations: Vec<_> = result
 664         .obligations
 665         .iter()
 666         .filter(|obligation| match obligation.predicate.kind() {
 667             // We found a `T: Foo<X = U>` predicate, let's check
 668             // if `U` references any unresolved type
 669             // variables. In principle, we only care if this
 670             // projection can help resolve any of the type
 671             // variables found in `result.value` -- but we just
 672             // check for any type variables here, for fear of
 673             // indirect obligations (e.g., we project to `?0`,
 674             // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
 675             // ?0>`).
 676             ty::PredicateKind::Projection(ref data) => {
 677                 infcx.unresolved_type_vars(&data.ty()).is_some()
 678             }
 679
 680             // We are only interested in `T: Foo<X = U>` predicates, whre
 681             // `U` references one of `unresolved_type_vars`. =)
 682             _ => false,
 683         })
 684         .cloned()
 685         .collect();
 686
 687     obligations.shrink_to_fit();
 688
 689     NormalizedTy { value: result.value, obligations }
 690 }
 691
 692 /// Whenever we give back a cache result for a projection like `<T as
 693 /// Trait>::Item ==> X`, we *always* include the obligation to prove
 694 /// that `T: Trait` (we may also include some other obligations). This
 695 /// may or may not be necessary -- in principle, all the obligations
 696 /// that must be proven to show that `T: Trait` were also returned
 697 /// when the cache was first populated. But there are some vague concerns,
 698 /// and so we take the precautionary measure of including `T: Trait` in
 699 /// the result:
 700 ///
 701 /// Concern #1. The current setup is fragile. Perhaps someone could
 702 /// have failed to prove the concerns from when the cache was
 703 /// populated, but also not have used a snapshot, in which case the
 704 /// cache could remain populated even though `T: Trait` has not been
 705 /// shown. In this case, the "other code" is at fault -- when you
 706 /// project something, you are supposed to either have a snapshot or
 707 /// else prove all the resulting obligations -- but it's still easy to
 708 /// get wrong.
 709 ///
 710 /// Concern #2. Even within the snapshot, if those original
 711 /// obligations are not yet proven, then we are able to do projections
 712 /// that may yet turn out to be wrong. This *may* lead to some sort
 713 /// of trouble, though we don't have a concrete example of how that
 714 /// can occur yet. But it seems risky at best.
 715 fn get_paranoid_cache_value_obligation<'a, 'tcx>(
 716     infcx: &'a InferCtxt<'a, 'tcx>,
 717     param_env: ty::ParamEnv<'tcx>,
 718     projection_ty: ty::ProjectionTy<'tcx>,
 719     cause: ObligationCause<'tcx>,
 720     depth: usize,
 721 ) -> PredicateObligation<'tcx> {
 722     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
 723     Obligation {
 724         cause,
 725         recursion_depth: depth,
 726         param_env,
 727         predicate: trait_ref.without_const().to_predicate(infcx.tcx),
 728     }
 729 }
 730
 731 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 732 /// hold. In various error cases, we cannot generate a valid
 733 /// normalized projection. Therefore, we create an inference variable
 734 /// return an associated obligation that, when fulfilled, will lead to
 735 /// an error.
 736 ///
 737 /// Note that we used to return `Error` here, but that was quite
 738 /// dubious -- the premise was that an error would *eventually* be
 739 /// reported, when the obligation was processed. But in general once
 740 /// you see a `Error` you are supposed to be able to assume that an
 741 /// error *has been* reported, so that you can take whatever heuristic
 742 /// paths you want to take. To make things worse, it was possible for
 743 /// cycles to arise, where you basically had a setup like `<MyType<$0>
 744 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
 745 /// Trait>::Foo> to `[type error]` would lead to an obligation of
 746 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
 747 /// an error for this obligation, but we legitimately should not,
 748 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 749 /// one case where this arose.)
 750 fn normalize_to_error<'a, 'tcx>(
 751     selcx: &mut SelectionContext<'a, 'tcx>,
 752     param_env: ty::ParamEnv<'tcx>,
 753     projection_ty: ty::ProjectionTy<'tcx>,
 754     cause: ObligationCause<'tcx>,
 755     depth: usize,
 756 ) -> NormalizedTy<'tcx> {
 757     let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
 758     let trait_obligation = Obligation {
 759         cause,
 760         recursion_depth: depth,
 761         param_env,
 762         predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
 763     };
 764     let tcx = selcx.infcx().tcx;
 765     let def_id = projection_ty.item_def_id;
 766     let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
 767         kind: TypeVariableOriginKind::NormalizeProjectionType,
 768         span: tcx.def_span(def_id),
 769     });
 770     Normalized { value: new_value, obligations: vec![trait_obligation] }
 771 }
 772
 773 enum ProjectedTy<'tcx> {
 774     Progress(Progress<'tcx>),
 775     NoProgress(Ty<'tcx>),
 776 }
 777
 778 struct Progress<'tcx> {
 779     ty: Ty<'tcx>,
 780     obligations: Vec<PredicateObligation<'tcx>>,
 781 }
 782
 783 impl<'tcx> Progress<'tcx> {
 784     fn error(tcx: TyCtxt<'tcx>) -> Self {
 785         Progress { ty: tcx.ty_error(), obligations: vec![] }
 786     }
 787
 788     fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
 789         debug!(
 790             "with_addl_obligations: self.obligations.len={} obligations.len={}",
 791             self.obligations.len(),
 792             obligations.len()
 793         );
 794
 795         debug!(
 796             "with_addl_obligations: self.obligations={:?} obligations={:?}",
 797             self.obligations, obligations
 798         );
 799
 800         self.obligations.append(&mut obligations);
 801         self
 802     }
 803 }
 804
 805 /// Computes the result of a projection type (if we can).
 806 ///
 807 /// IMPORTANT:
 808 /// - `obligation` must be fully normalized
 809 fn project_type<'cx, 'tcx>(
 810     selcx: &mut SelectionContext<'cx, 'tcx>,
 811     obligation: &ProjectionTyObligation<'tcx>,
 812 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
 813     debug!("project(obligation={:?})", obligation);
 814
 815     if !selcx.tcx().sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
 816         debug!("project: overflow!");
 817         return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
 818     }
 819
 820     let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
 821
 822     debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
 823
 824     if obligation_trait_ref.references_error() {
 825         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
 826     }
 827
 828     let mut candidates = ProjectionTyCandidateSet::None;
 829
 830     // Make sure that the following procedures are kept in order. ParamEnv
 831     // needs to be first because it has highest priority, and Select checks
 832     // the return value of push_candidate which assumes it's ran at last.
 833     assemble_candidates_from_param_env(selcx, obligation, &obligation_trait_ref, &mut candidates);
 834
 835     assemble_candidates_from_trait_def(selcx, obligation, &obligation_trait_ref, &mut candidates);
 836
 837     assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
 838
 839     match candidates {
 840         ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
 841             confirm_candidate(selcx, obligation, &obligation_trait_ref, candidate),
 842         )),
 843         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
 844             selcx
 845                 .tcx()
 846                 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs),
 847         )),
 848         // Error occurred while trying to processing impls.
 849         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
 850         // Inherent ambiguity that prevents us from even enumerating the
 851         // candidates.
 852         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
 853     }
 854 }
 855
 856 /// The first thing we have to do is scan through the parameter
 857 /// environment to see whether there are any projection predicates
 858 /// there that can answer this question.
 859 fn assemble_candidates_from_param_env<'cx, 'tcx>(
 860     selcx: &mut SelectionContext<'cx, 'tcx>,
 861     obligation: &ProjectionTyObligation<'tcx>,
 862     obligation_trait_ref: &ty::TraitRef<'tcx>,
 863     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 864 ) {
 865     debug!("assemble_candidates_from_param_env(..)");
 866     assemble_candidates_from_predicates(
 867         selcx,
 868         obligation,
 869         obligation_trait_ref,
 870         candidate_set,
 871         ProjectionTyCandidate::ParamEnv,
 872         obligation.param_env.caller_bounds().iter(),
 873     );
 874 }
 875
 876 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
 877 /// that the definition of `Foo` has some clues:
 878 ///
 879 /// ```
 880 /// trait Foo {
 881 ///     type FooT : Bar<BarT=i32>
 882 /// }
 883 /// ```
 884 ///
 885 /// Here, for example, we could conclude that the result is `i32`.
 886 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
 887     selcx: &mut SelectionContext<'cx, 'tcx>,
 888     obligation: &ProjectionTyObligation<'tcx>,
 889     obligation_trait_ref: &ty::TraitRef<'tcx>,
 890     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 891 ) {
 892     debug!("assemble_candidates_from_trait_def(..)");
 893
 894     let tcx = selcx.tcx();
 895     // Check whether the self-type is itself a projection.
 896     // If so, extract what we know from the trait and try to come up with a good answer.
 897     let bounds = match obligation_trait_ref.self_ty().kind {
 898         ty::Projection(ref data) => {
 899             tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
 900         }
 901         ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
 902         ty::Infer(ty::TyVar(_)) => {
 903             // If the self-type is an inference variable, then it MAY wind up
 904             // being a projected type, so induce an ambiguity.
 905             candidate_set.mark_ambiguous();
 906             return;
 907         }
 908         _ => return,
 909     };
 910
 911     assemble_candidates_from_predicates(
 912         selcx,
 913         obligation,
 914         obligation_trait_ref,
 915         candidate_set,
 916         ProjectionTyCandidate::TraitDef,
 917         bounds.iter(),
 918     )
 919 }
 920
 921 fn assemble_candidates_from_predicates<'cx, 'tcx>(
 922     selcx: &mut SelectionContext<'cx, 'tcx>,
 923     obligation: &ProjectionTyObligation<'tcx>,
 924     obligation_trait_ref: &ty::TraitRef<'tcx>,
 925     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 926     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
 927     env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 928 ) {
 929     debug!("assemble_candidates_from_predicates(obligation={:?})", obligation);
 930     let infcx = selcx.infcx();
 931     for predicate in env_predicates {
 932         debug!("assemble_candidates_from_predicates: predicate={:?}", predicate);
 933         if let &ty::PredicateKind::Projection(data) = predicate.kind() {
 934             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
 935
 936             let is_match = same_def_id
 937                 && infcx.probe(|_| {
 938                     let data_poly_trait_ref = data.to_poly_trait_ref(infcx.tcx);
 939                     let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
 940                     infcx
 941                         .at(&obligation.cause, obligation.param_env)
 942                         .sup(obligation_poly_trait_ref, data_poly_trait_ref)
 943                         .map(|InferOk { obligations: _, value: () }| {
 944                             // FIXME(#32730) -- do we need to take obligations
 945                             // into account in any way? At the moment, no.
 946                         })
 947                         .is_ok()
 948                 });
 949
 950             debug!(
 951                 "assemble_candidates_from_predicates: candidate={:?} \
 952                  is_match={} same_def_id={}",
 953                 data, is_match, same_def_id
 954             );
 955
 956             if is_match {
 957                 candidate_set.push_candidate(ctor(data));
 958             }
 959         }
 960     }
 961 }
 962
 963 fn assemble_candidates_from_impls<'cx, 'tcx>(
 964     selcx: &mut SelectionContext<'cx, 'tcx>,
 965     obligation: &ProjectionTyObligation<'tcx>,
 966     obligation_trait_ref: &ty::TraitRef<'tcx>,
 967     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 968 ) {
 969     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
 970     // start out by selecting the predicate `T as TraitRef<...>`:
 971     let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
 972     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
 973     let _ = selcx.infcx().commit_if_ok(|_| {
 974         let impl_source = match selcx.select(&trait_obligation) {
 975             Ok(Some(impl_source)) => impl_source,
 976             Ok(None) => {
 977                 candidate_set.mark_ambiguous();
 978                 return Err(());
 979             }
 980             Err(e) => {
 981                 debug!("assemble_candidates_from_impls: selection error {:?}", e);
 982                 candidate_set.mark_error(e);
 983                 return Err(());
 984             }
 985         };
 986
 987         let eligible = match &impl_source {
 988             super::ImplSourceClosure(_)
 989             | super::ImplSourceGenerator(_)
 990             | super::ImplSourceFnPointer(_)
 991             | super::ImplSourceObject(_)
 992             | super::ImplSourceTraitAlias(_) => {
 993                 debug!("assemble_candidates_from_impls: impl_source={:?}", impl_source);
 994                 true
 995             }
 996             super::ImplSourceUserDefined(impl_data) => {
 997                 // We have to be careful when projecting out of an
 998                 // impl because of specialization. If we are not in
 999                 // codegen (i.e., projection mode is not "any"), and the
1000                 // impl's type is declared as default, then we disable
1001                 // projection (even if the trait ref is fully
1002                 // monomorphic). In the case where trait ref is not
1003                 // fully monomorphic (i.e., includes type parameters),
1004                 // this is because those type parameters may
1005                 // ultimately be bound to types from other crates that
1006                 // may have specialized impls we can't see. In the
1007                 // case where the trait ref IS fully monomorphic, this
1008                 // is a policy decision that we made in the RFC in
1009                 // order to preserve flexibility for the crate that
1010                 // defined the specializable impl to specialize later
1011                 // for existing types.
1012                 //
1013                 // In either case, we handle this by not adding a
1014                 // candidate for an impl if it contains a `default`
1015                 // type.
1016                 //
1017                 // NOTE: This should be kept in sync with the similar code in
1018                 // `rustc_ty::instance::resolve_associated_item()`.
1019                 let node_item =
1020                     assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1021                         .map_err(|ErrorReported| ())?;
1022
1023                 if node_item.is_final() {
1024                     // Non-specializable items are always projectable.
1025                     true
1026                 } else {
1027                     // Only reveal a specializable default if we're past type-checking
1028                     // and the obligation is monomorphic, otherwise passes such as
1029                     // transmute checking and polymorphic MIR optimizations could
1030                     // get a result which isn't correct for all monomorphizations.
1031                     if obligation.param_env.reveal() == Reveal::All {
1032                         // NOTE(eddyb) inference variables can resolve to parameters, so
1033                         // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1034                         let poly_trait_ref =
1035                             selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
1036                         !poly_trait_ref.still_further_specializable()
1037                     } else {
1038                         debug!(
1039                             "assemble_candidates_from_impls: not eligible due to default: \
1040                              assoc_ty={} predicate={}",
1041                             selcx.tcx().def_path_str(node_item.item.def_id),
1042                             obligation.predicate,
1043                         );
1044                         false
1045                     }
1046                 }
1047             }
1048             super::ImplSourceDiscriminantKind(..) => {
1049                 // While `DiscriminantKind` is automatically implemented for every type,
1050                 // the concrete discriminant may not be known yet.
1051                 //
1052                 // Any type with multiple potential discriminant types is therefore not eligible.
1053                 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1054
1055                 match self_ty.kind {
1056                     ty::Bool
1057                     | ty::Char
1058                     | ty::Int(_)
1059                     | ty::Uint(_)
1060                     | ty::Float(_)
1061                     | ty::Adt(..)
1062                     | ty::Foreign(_)
1063                     | ty::Str
1064                     | ty::Array(..)
1065                     | ty::Slice(_)
1066                     | ty::RawPtr(..)
1067                     | ty::Ref(..)
1068                     | ty::FnDef(..)
1069                     | ty::FnPtr(..)
1070                     | ty::Dynamic(..)
1071                     | ty::Closure(..)
1072                     | ty::Generator(..)
1073                     | ty::GeneratorWitness(..)
1074                     | ty::Never
1075                     | ty::Tuple(..)
1076                     // Integers and floats always have `u8` as their discriminant.
1077                     | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1078
1079                     ty::Projection(..)
1080                     | ty::Opaque(..)
1081                     | ty::Param(..)
1082                     | ty::Bound(..)
1083                     | ty::Placeholder(..)
1084                     | ty::Infer(..)
1085                     | ty::Error(_) => false,
1086                 }
1087             }
1088             super::ImplSourceParam(..) => {
1089                 // This case tell us nothing about the value of an
1090                 // associated type. Consider:
1091                 //
1092                 // ```
1093                 // trait SomeTrait { type Foo; }
1094                 // fn foo<T:SomeTrait>(...) { }
1095                 // ```
1096                 //
1097                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1098                 // : SomeTrait` binding does not help us decide what the
1099                 // type `Foo` is (at least, not more specifically than
1100                 // what we already knew).
1101                 //
1102                 // But wait, you say! What about an example like this:
1103                 //
1104                 // ```
1105                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1106                 // ```
1107                 //
1108                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1109                 // resolve `T::Foo`? And of course it does, but in fact
1110                 // that single predicate is desugared into two predicates
1111                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1112                 // projection. And the projection where clause is handled
1113                 // in `assemble_candidates_from_param_env`.
1114                 false
1115             }
1116             super::ImplSourceAutoImpl(..) | super::ImplSourceBuiltin(..) => {
1117                 // These traits have no associated types.
1118                 span_bug!(
1119                     obligation.cause.span,
1120                     "Cannot project an associated type from `{:?}`",
1121                     impl_source
1122                 );
1123             }
1124         };
1125
1126         if eligible {
1127             if candidate_set.push_candidate(ProjectionTyCandidate::Select(impl_source)) {
1128                 Ok(())
1129             } else {
1130                 Err(())
1131             }
1132         } else {
1133             Err(())
1134         }
1135     });
1136 }
1137
1138 fn confirm_candidate<'cx, 'tcx>(
1139     selcx: &mut SelectionContext<'cx, 'tcx>,
1140     obligation: &ProjectionTyObligation<'tcx>,
1141     obligation_trait_ref: &ty::TraitRef<'tcx>,
1142     candidate: ProjectionTyCandidate<'tcx>,
1143 ) -> Progress<'tcx> {
1144     debug!("confirm_candidate(candidate={:?}, obligation={:?})", candidate, obligation);
1145
1146     let mut progress = match candidate {
1147         ProjectionTyCandidate::ParamEnv(poly_projection)
1148         | ProjectionTyCandidate::TraitDef(poly_projection) => {
1149             confirm_param_env_candidate(selcx, obligation, poly_projection)
1150         }
1151
1152         ProjectionTyCandidate::Select(impl_source) => {
1153             confirm_select_candidate(selcx, obligation, obligation_trait_ref, impl_source)
1154         }
1155     };
1156     // When checking for cycle during evaluation, we compare predicates with
1157     // "syntactic" equality. Since normalization generally introduces a type
1158     // with new region variables, we need to resolve them to existing variables
1159     // when possible for this to work. See `auto-trait-projection-recursion.rs`
1160     // for a case where this matters.
1161     if progress.ty.has_infer_regions() {
1162         progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
1163     }
1164     progress
1165 }
1166
1167 fn confirm_select_candidate<'cx, 'tcx>(
1168     selcx: &mut SelectionContext<'cx, 'tcx>,
1169     obligation: &ProjectionTyObligation<'tcx>,
1170     obligation_trait_ref: &ty::TraitRef<'tcx>,
1171     impl_source: Selection<'tcx>,
1172 ) -> Progress<'tcx> {
1173     match impl_source {
1174         super::ImplSourceUserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1175         super::ImplSourceGenerator(data) => confirm_generator_candidate(selcx, obligation, data),
1176         super::ImplSourceClosure(data) => confirm_closure_candidate(selcx, obligation, data),
1177         super::ImplSourceFnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1178         super::ImplSourceDiscriminantKind(data) => {
1179             confirm_discriminant_kind_candidate(selcx, obligation, data)
1180         }
1181         super::ImplSourceObject(_) => {
1182             confirm_object_candidate(selcx, obligation, obligation_trait_ref)
1183         }
1184         super::ImplSourceAutoImpl(..)
1185         | super::ImplSourceParam(..)
1186         | super::ImplSourceBuiltin(..)
1187         | super::ImplSourceTraitAlias(..) =>
1188         // we don't create Select candidates with this kind of resolution
1189         {
1190             span_bug!(
1191                 obligation.cause.span,
1192                 "Cannot project an associated type from `{:?}`",
1193                 impl_source
1194             )
1195         }
1196     }
1197 }
1198
1199 fn confirm_object_candidate<'cx, 'tcx>(
1200     selcx: &mut SelectionContext<'cx, 'tcx>,
1201     obligation: &ProjectionTyObligation<'tcx>,
1202     obligation_trait_ref: &ty::TraitRef<'tcx>,
1203 ) -> Progress<'tcx> {
1204     let self_ty = obligation_trait_ref.self_ty();
1205     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1206     debug!("confirm_object_candidate(object_ty={:?})", object_ty);
1207     let data = match object_ty.kind {
1208         ty::Dynamic(ref data, ..) => data,
1209         _ => span_bug!(
1210             obligation.cause.span,
1211             "confirm_object_candidate called with non-object: {:?}",
1212             object_ty
1213         ),
1214     };
1215     let env_predicates = data
1216         .projection_bounds()
1217         .map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate(selcx.tcx()));
1218     let env_predicate = {
1219         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1220
1221         // select only those projections that are actually projecting an
1222         // item with the correct name
1223         let env_predicates = env_predicates.filter_map(|o| match o.predicate.kind() {
1224             &ty::PredicateKind::Projection(data)
1225                 if data.projection_def_id() == obligation.predicate.item_def_id =>
1226             {
1227                 Some(data)
1228             }
1229             _ => None,
1230         });
1231
1232         // select those with a relevant trait-ref
1233         let mut env_predicates = env_predicates.filter(|data| {
1234             let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1235             let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1236             selcx.infcx().probe(|_| {
1237                 selcx
1238                     .infcx()
1239                     .at(&obligation.cause, obligation.param_env)
1240                     .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1241                     .is_ok()
1242             })
1243         });
1244
1245         // select the first matching one; there really ought to be one or
1246         // else the object type is not WF, since an object type should
1247         // include all of its projections explicitly
1248         match env_predicates.next() {
1249             Some(env_predicate) => env_predicate,
1250             None => {
1251                 debug!(
1252                     "confirm_object_candidate: no env-predicate \
1253                      found in object type `{:?}`; ill-formed",
1254                     object_ty
1255                 );
1256                 return Progress::error(selcx.tcx());
1257             }
1258         }
1259     };
1260
1261     confirm_param_env_candidate(selcx, obligation, env_predicate)
1262 }
1263
1264 fn confirm_generator_candidate<'cx, 'tcx>(
1265     selcx: &mut SelectionContext<'cx, 'tcx>,
1266     obligation: &ProjectionTyObligation<'tcx>,
1267     impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1268 ) -> Progress<'tcx> {
1269     let gen_sig = impl_source.substs.as_generator().poly_sig();
1270     let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1271         selcx,
1272         obligation.param_env,
1273         obligation.cause.clone(),
1274         obligation.recursion_depth + 1,
1275         &gen_sig,
1276     );
1277
1278     debug!(
1279         "confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1280         obligation, gen_sig, obligations
1281     );
1282
1283     let tcx = selcx.tcx();
1284
1285     let gen_def_id = tcx.require_lang_item(GeneratorTraitLangItem, None);
1286
1287     let predicate = super::util::generator_trait_ref_and_outputs(
1288         tcx,
1289         gen_def_id,
1290         obligation.predicate.self_ty(),
1291         gen_sig,
1292     )
1293     .map_bound(|(trait_ref, yield_ty, return_ty)| {
1294         let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1295         let ty = if name == sym::Return {
1296             return_ty
1297         } else if name == sym::Yield {
1298             yield_ty
1299         } else {
1300             bug!()
1301         };
1302
1303         ty::ProjectionPredicate {
1304             projection_ty: ty::ProjectionTy {
1305                 substs: trait_ref.substs,
1306                 item_def_id: obligation.predicate.item_def_id,
1307             },
1308             ty,
1309         }
1310     });
1311
1312     confirm_param_env_candidate(selcx, obligation, predicate)
1313         .with_addl_obligations(impl_source.nested)
1314         .with_addl_obligations(obligations)
1315 }
1316
1317 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1318     selcx: &mut SelectionContext<'cx, 'tcx>,
1319     obligation: &ProjectionTyObligation<'tcx>,
1320     _: ImplSourceDiscriminantKindData,
1321 ) -> Progress<'tcx> {
1322     let tcx = selcx.tcx();
1323
1324     let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1325     let substs = tcx.mk_substs([self_ty.into()].iter());
1326
1327     let assoc_items = tcx.associated_items(tcx.lang_items().discriminant_kind_trait().unwrap());
1328     // FIXME: emit an error if the trait definition is wrong
1329     let discriminant_def_id = assoc_items.in_definition_order().next().unwrap().def_id;
1330
1331     let discriminant_ty = match self_ty.kind {
1332         // Use the discriminant type for enums.
1333         ty::Adt(adt, _) if adt.is_enum() => adt.repr.discr_type().to_ty(tcx),
1334         // Default to `i32` for generators.
1335         ty::Generator(..) => tcx.types.i32,
1336         // Use `u8` for all other types.
1337         _ => tcx.types.u8,
1338     };
1339
1340     let predicate = ty::ProjectionPredicate {
1341         projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1342         ty: discriminant_ty,
1343     };
1344
1345     confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate))
1346 }
1347
1348 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1349     selcx: &mut SelectionContext<'cx, 'tcx>,
1350     obligation: &ProjectionTyObligation<'tcx>,
1351     fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1352 ) -> Progress<'tcx> {
1353     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1354     let sig = fn_type.fn_sig(selcx.tcx());
1355     let Normalized { value: sig, obligations } = normalize_with_depth(
1356         selcx,
1357         obligation.param_env,
1358         obligation.cause.clone(),
1359         obligation.recursion_depth + 1,
1360         &sig,
1361     );
1362
1363     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1364         .with_addl_obligations(fn_pointer_impl_source.nested)
1365         .with_addl_obligations(obligations)
1366 }
1367
1368 fn confirm_closure_candidate<'cx, 'tcx>(
1369     selcx: &mut SelectionContext<'cx, 'tcx>,
1370     obligation: &ProjectionTyObligation<'tcx>,
1371     impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1372 ) -> Progress<'tcx> {
1373     let closure_sig = impl_source.substs.as_closure().sig();
1374     let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1375         selcx,
1376         obligation.param_env,
1377         obligation.cause.clone(),
1378         obligation.recursion_depth + 1,
1379         &closure_sig,
1380     );
1381
1382     debug!(
1383         "confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1384         obligation, closure_sig, obligations
1385     );
1386
1387     confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1388         .with_addl_obligations(impl_source.nested)
1389         .with_addl_obligations(obligations)
1390 }
1391
1392 fn confirm_callable_candidate<'cx, 'tcx>(
1393     selcx: &mut SelectionContext<'cx, 'tcx>,
1394     obligation: &ProjectionTyObligation<'tcx>,
1395     fn_sig: ty::PolyFnSig<'tcx>,
1396     flag: util::TupleArgumentsFlag,
1397 ) -> Progress<'tcx> {
1398     let tcx = selcx.tcx();
1399
1400     debug!("confirm_callable_candidate({:?},{:?})", obligation, fn_sig);
1401
1402     let fn_once_def_id = tcx.require_lang_item(FnOnceTraitLangItem, None);
1403     let fn_once_output_def_id = tcx.require_lang_item(FnOnceOutputLangItem, None);
1404
1405     let predicate = super::util::closure_trait_ref_and_return_type(
1406         tcx,
1407         fn_once_def_id,
1408         obligation.predicate.self_ty(),
1409         fn_sig,
1410         flag,
1411     )
1412     .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1413         projection_ty: ty::ProjectionTy {
1414             substs: trait_ref.substs,
1415             item_def_id: fn_once_output_def_id,
1416         },
1417         ty: ret_type,
1418     });
1419
1420     confirm_param_env_candidate(selcx, obligation, predicate)
1421 }
1422
1423 fn confirm_param_env_candidate<'cx, 'tcx>(
1424     selcx: &mut SelectionContext<'cx, 'tcx>,
1425     obligation: &ProjectionTyObligation<'tcx>,
1426     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1427 ) -> Progress<'tcx> {
1428     let infcx = selcx.infcx();
1429     let cause = &obligation.cause;
1430     let param_env = obligation.param_env;
1431
1432     let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1433         cause.span,
1434         LateBoundRegionConversionTime::HigherRankedType,
1435         &poly_cache_entry,
1436     );
1437
1438     let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1439     let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1440     match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1441         Ok(InferOk { value: _, obligations }) => Progress { ty: cache_entry.ty, obligations },
1442         Err(e) => {
1443             let msg = format!(
1444                 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1445                 obligation, poly_cache_entry, e,
1446             );
1447             debug!("confirm_param_env_candidate: {}", msg);
1448             let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1449             Progress { ty: err, obligations: vec![] }
1450         }
1451     }
1452 }
1453
1454 fn confirm_impl_candidate<'cx, 'tcx>(
1455     selcx: &mut SelectionContext<'cx, 'tcx>,
1456     obligation: &ProjectionTyObligation<'tcx>,
1457     impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1458 ) -> Progress<'tcx> {
1459     let tcx = selcx.tcx();
1460
1461     let ImplSourceUserDefinedData { impl_def_id, substs, nested } = impl_impl_source;
1462     let assoc_item_id = obligation.predicate.item_def_id;
1463     let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1464
1465     let param_env = obligation.param_env;
1466     let assoc_ty = match assoc_ty_def(selcx, impl_def_id, assoc_item_id) {
1467         Ok(assoc_ty) => assoc_ty,
1468         Err(ErrorReported) => return Progress { ty: tcx.ty_error(), obligations: nested },
1469     };
1470
1471     if !assoc_ty.item.defaultness.has_value() {
1472         // This means that the impl is missing a definition for the
1473         // associated type. This error will be reported by the type
1474         // checker method `check_impl_items_against_trait`, so here we
1475         // just return Error.
1476         debug!(
1477             "confirm_impl_candidate: no associated type {:?} for {:?}",
1478             assoc_ty.item.ident, obligation.predicate
1479         );
1480         return Progress { ty: tcx.ty_error(), obligations: nested };
1481     }
1482     // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1483     //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1484     //
1485     // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1486     // * `substs` is `[u32]`
1487     // * `substs` ends up as `[u32, S]`
1488     let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1489     let substs =
1490         translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1491     let ty = tcx.type_of(assoc_ty.item.def_id);
1492     if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1493         let err = tcx.ty_error_with_message(
1494             DUMMY_SP,
1495             "impl item and trait item have different parameter counts",
1496         );
1497         Progress { ty: err, obligations: nested }
1498     } else {
1499         Progress { ty: ty.subst(tcx, substs), obligations: nested }
1500     }
1501 }
1502
1503 /// Locate the definition of an associated type in the specialization hierarchy,
1504 /// starting from the given impl.
1505 ///
1506 /// Based on the "projection mode", this lookup may in fact only examine the
1507 /// topmost impl. See the comments for `Reveal` for more details.
1508 fn assoc_ty_def(
1509     selcx: &SelectionContext<'_, '_>,
1510     impl_def_id: DefId,
1511     assoc_ty_def_id: DefId,
1512 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
1513     let tcx = selcx.tcx();
1514     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1515     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1516     let trait_def = tcx.trait_def(trait_def_id);
1517
1518     // This function may be called while we are still building the
1519     // specialization graph that is queried below (via TraitDef::ancestors()),
1520     // so, in order to avoid unnecessary infinite recursion, we manually look
1521     // for the associated item at the given impl.
1522     // If there is no such item in that impl, this function will fail with a
1523     // cycle error if the specialization graph is currently being built.
1524     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1525     for item in impl_node.items(tcx) {
1526         if matches!(item.kind, ty::AssocKind::Type)
1527             && tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id)
1528         {
1529             return Ok(specialization_graph::LeafDef {
1530                 item: *item,
1531                 defining_node: impl_node,
1532                 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1533             });
1534         }
1535     }
1536
1537     let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1538     if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
1539         Ok(assoc_item)
1540     } else {
1541         // This is saying that neither the trait nor
1542         // the impl contain a definition for this
1543         // associated type.  Normally this situation
1544         // could only arise through a compiler bug --
1545         // if the user wrote a bad item name, it
1546         // should have failed in astconv.
1547         bug!("No associated type `{}` for {}", assoc_ty_name, tcx.def_path_str(impl_def_id))
1548     }
1549 }
1550
1551 crate trait ProjectionCacheKeyExt<'tcx>: Sized {
1552     fn from_poly_projection_predicate(
1553         selcx: &mut SelectionContext<'cx, 'tcx>,
1554         predicate: ty::PolyProjectionPredicate<'tcx>,
1555     ) -> Option<Self>;
1556 }
1557
1558 impl<'tcx> ProjectionCacheKeyExt<'tcx> for ProjectionCacheKey<'tcx> {
1559     fn from_poly_projection_predicate(
1560         selcx: &mut SelectionContext<'cx, 'tcx>,
1561         predicate: ty::PolyProjectionPredicate<'tcx>,
1562     ) -> Option<Self> {
1563         let infcx = selcx.infcx();
1564         // We don't do cross-snapshot caching of obligations with escaping regions,
1565         // so there's no cache key to use
1566         predicate.no_bound_vars().map(|predicate| {
1567             ProjectionCacheKey::new(
1568                 // We don't attempt to match up with a specific type-variable state
1569                 // from a specific call to `opt_normalize_projection_type` - if
1570                 // there's no precise match, the original cache entry is "stranded"
1571                 // anyway.
1572                 infcx.resolve_vars_if_possible(&predicate.projection_ty),
1573             )
1574         })
1575     }
1576 }